Electrolytic system for recovering metal from chemical solutions with controlled plating current

ABSTRACT

An electrolytic plating system in which the plating current through the cell is controlled in proportion to the metal ion concentration in the solution by means of an integrator circuit which increases and decreases amperage respectively as a function of the amount of metal ion bearing material added to the solution and the amount of metal ion leaving the solution by being plated upon the cathode of the cell. Improved apparatus for circulating the solution within the cell includes a specially adapted centrifugal impeller. An embodiment of the system is described in relation to silver recovery from photographic fixing solutions.

United States Patent 1191 Cave [ ELECTROLYTTC SYSTEM FOR RECOVERING METAL FROM CHEMICAL SOLUTIONS WITH CONTROLLED PLATING CURRENT [75] Inventor:

[52] US. Cl. 204/229; 204/109; 204/218; 204/228; 204/273 Int. Cl. C25C 7/00 Field of Search 204/109, 228, 229, 218

References Cited UNITED STATES PATENTS 3/1933 Hickman 204/109 12/1968 Wick ct a1. 204/228 X 8/1970 Engelman 204/228 12/1970 Snook 8t alum 204/228 10/197] Favell et al. 204/228 INTEGRATOR MANUAL ADJUST CKT Dec. 9, 1975 3,658,683 4/1972 Lagier et =11 1, 204/229 x 3,694,341 9/1972 Luck, Jr l 1 1 11 204/109 x 3,715,291 2/1973 Bentley .1 204/229 x Primary ExaminerJohn H. Mack Assistant ExaminerW. 1. Solomon Attorney, Agent, or Firm-Lane, Aitken, Dunner & Ziems [57] ABSTRACT An electrolytic plating system in which the plating current through the cell is controlled in proportion to the metal ion concentration in the solution by means of an integrator circuit which increases and decreases amperage respectively as a function of the amount of metal ion bearing material added to the solution and the amount of metal ion leaving the solution by being plated upon the cathode of the cell. Improved apparatus for circulating the solution within the cell includes a specially adapted centrifugal impeller. An embodiment of the system is described in relation to silver recovery from photographic fixing solutions,

18 Claims, 8 Drawing Figures D WORKING |O2 POWER VOLTAGE 1 SUPPLY TO ALL CUVTRQL CIRCUITS PLATlNG CURRENT 68 i a0 16 SENSOR INTEGRATOR nocessoa CONTROL FEED $12 151 CIRCUIT SWITCH CKT RESET a2 a4 STANDBY 74 76 CIRCUlT SHUTDOWN STANDBY CIRCUIT walnut:

PROCESSING I i swear FAILURE LOADNG DISPLAY F 94 ALARM l N6T=L OVI T 7 7 7 CE LL RROR 7 FROM CELL FAIL OVERRIDE 1 CELL FLOW I I6 I SENSOR 94 FAILURE CELL 1- FAIL l 0e TECTION o 1 SHORT on l: 90 (:ATll-ooe SENSOR 96 DETECTOR CELL 153% OPEN 99 US. Patent Dec. 9, 1975 Sheet 1 of5 3,925,184

FIG I ELECTROLYTIC SYSTEM FOR RECOVERING METAL FROM CHEMICAL SOLUTIONS WITH CONTROLLED PLATING CURRENT BACKGROUND OF THE INVENTION In the processing of photographic film, chemical fixing solutions eventually become exhausted due to chemical reaction with material on the film, in particular, silver halide, with the fixer. The higher the concentration of silver salts in the fixing solution, the lower the quality of the processed photograph. In bulk processing of x-ray film, for example, the quantities of film bearing silver halide added to the fixing solution are frequently so high that a given fixing solution may loose its effectiveness in rendering high quality photographic processing and thus become exhausted in a relatively short period of time. When this occurs, the fixing solution must be discarded. The Environmental Protection Agency has set a limit on the concentration of silver salts which may be discharged into the public sewer system as waste because of the possibility of contamination. In addition to the contamination problem, the rapid exhaustion of fixing solutions results in more frequent procurement of fresh fixing solutions and loss of the monetary value of the silver carried in the exhausted solution. Accordingly, if the silver salt can be continuously removed from the fixing solution while photographic processing is carried on, the life of the solution can be greatly extended, eliminating the cost and contamination problems in accordance with the efficiency of the removal method.

One method for effecting silver ion removal is to use an electrolytic cell, which has the added advantage of recovering pure silver metal in a saleable form to further reduce the net cost of high quality photographic processing. In used photographic fixing solutions, the metal salt, silver thiosulfate, is present in both the primary and secondary dissociated state:

Ag*S"O :tAgSO Ag*(Primary) (l) AgSmH SO Ag (Secondary) (2) This dissociation leaves the silver ion free to accept an electron at the cathode:

Ag e Ag (3) However, the thiosulfate ion will also accept electrons at the cathode such that:

s o= 2e 1 s=*=+ S= (4) However, if the silver ion is present, equation 3 is the preferential reaction. If the sulfide ion is produced, it will combine with silver ions,

producing the precipitate silver sulfide which is an unwanted reaction. The sulfide ion will also combine with hydrogen to produce hydrogen sulfide gas:

2HzO+S H1ST+2OH-0r2H +S H ST (5] The production of hydrogen sulfide and silver sulfide destroys the solution, and the film processing apparatus must be cleaned before fresh solution can be added.

Prior art devices, like that shown in U.S. Pat. No. 3,694,341 issued to William R. Luck, Jr., Sept. 26, I972, contained no means for automatically varying the length of plating time or the plating current through the electrolytic cell. If the plating current remains constant and the silver ion concentration is reduced by plating silver out on the cathode, the unwanted reaction, equation (4), becomes the preferential reaction and the sulfide ion is produced in quantity. The sulfide ions caused by excessive plating current result in the production of hydrogen sulfide and silver sulfide lessening the the quality of the solution and of the silver plated out. On the other hand, if the plating current remains constant and the silver ion concentration is increased, for example, by adding more film to the processor, the original problem obtains of failing to re- 0 move enough silver ions from the solution. lnsufficient plating current results in loss of pure silver as well as a high silver ion concentration in the solution which reduces the effectiveness of the fixing solution for photographic purposes and increases the contamination of sewer systems when the solution is discarded.

One control system is known in the prior art for adjusting the voltage across the cell when a photocell detects that the reflectivity of the silver is decreasing due to acquisition by the silver being plated out of a brownish color because of reaction with the sulfide ions. But this system could work only within marginally acceptable limits, and in any event, the plating current would rarely have been optimized, if ever, in such a system. Moreover, where voltage, not current, is being controlled, the plating current is only partly controlled because it remains a function of the resistive load of the cell. In order to inhibit the production of sulfide ions characteristic of excessive plating current, prior devices have employed stirring means to promote homogeneity of the silver ion concentration throughout the cell and to prevent the depletion of silver ions near the cathode. These stirring means include curved rods and paddles to agitate the solution and/or external pumps to circulate the liquid within the cell. Voids or areas of relatively low metal ion concentration within the solution resulted because of the high rate of silver ion removal near the cathode and eddy current flow paths characteristic of such devices. Such voids necessitated less than optimum plating current thorugh the cell to prevent production of sulfide ions near the cathode.

SUMMARY OF THE INVENTION The general purpose of the invention is to increase the efficiency of electrolytic metal ion removal systems by controlling the level of plating current and the plat ing time in response to sensed parmeters which in combination provide a means for determining the instantaneous concentration of silver ions in a given solution. A connected object of the invention is to improve the means for circulating the solution through the electrolytic cell.

According to the invention, an electronic integrator circuit controls the level of plating current through the electrolytic cell as a function of silver ion concentration. This system is based on an assumption that the optimum plating current in the system at any given time is a linear function of silver ion concentration represented by the equation I =Ag(c/v), where small v is the volume of the fixing solution, c is a constant dependent upon the rate of dissociation of the silver salt and Ag is the total amount of silver in the system. When silver leaves the system by being plated out, the current through the cell must be reduced such that the previous equation remains balanced. The rate of removal of silver is stated by Faradays law dAg/dr KI, where K is Faradays constant for silver. The silver concentration, and thus the plating current as a function of time, is the solution of the above differential equation, which is as follows:

Ag I,,( vlc)e"""' where I U/c) is the initial current through the cell for an initial concentration and t is time.

During the time that silver ions are introduced at a uniform rate into the solution, for example, by advancing new film into the processor, a reference signal is generated. The time integral of this reference signal is calculated to represent the amount of silver added to the solution. On the other hand, as the silver is plated out and the silver ion concentration is reduced, a sec ond reference signal, representing the instantaneous level of plating current, is generated. The time integral of this second reference signal is subtracted from the first integral. The resultant calculation represents the instantaneous quantity of silver in solution. The variable current is controlled by the result of this calculation. Hence, the plating current through the cell is increased in proportion to the quantity of metal ions that are added and decreased in proportion to the quantity of metal ions that are removed. If the rate at which metal ions are being added and removed is precisely the same, the quantity of ions in solution would remain the same and the plating current through the cell would be kept constant since the metal ion concentration would be constant.

In the preferred embodiment, an analog integrator is used to control the variable current source. The analog integrator includes an operational amplifier with capacitive feedback provided by an electrolytic capacitive element with special nonlinear memory properties. When a constant current is applied to the capacitive element, the voltage across the plates increases linearly as a function of time, until a particular voltage is reached, and then maintains that voltage even through the constant current is still being applied. The capacitive element will retain the maximum voltage until the same amount of charge has been removed as was added to it after reaching the maximum holding voltage. This holding voltage is made to correspond to the maximum current possible from the variable current source. This characteristic allows the electrolytic cell to operate at the systems maximum efficiency regardless of the current limitation inherent in the system.

In the mechanical portion of the metal recovery system, a centrifugal impeller is mounted in the electrolytic cell and connected to an external motor by a horizontal shaft through one sidewall of the cell. Rotation of the impeller causes ion laden solution entering the electrolytic cell through a hollow anode to be drawn into orifices formed in the impeller and to be discharged through offset ducts in the impeller by centrifugal force, outwardly toward a coaxial cylindrical cathode, creating a homogenous solution about the cathode surface. The action of the liquid in turn creates a vortex with a characteristic pressure differential from the center to the inner side wall of the cathode causing ion free solution to exit through a port near the inner side wall of the cathode. The centrifugal impeller represents an improvement over the multi-bladed impeller disclosed in the above-mentioned patent.

In the control system, when the output of the integrator is reduced to a predetermined minimum level corresponding to a zero concentration of silver ion in the solution, a standby system stops the motor to save wear on the bearings and sets the integrator to zero halting the plating current to prevent accumulated errors in the integrator output after a number of such standby conditions have occurred. When more film, for example, is added to the processor, the first reference signal to the integrator is resumed, with a concomitant resumption of proportional plating current through the electrolytic cell.

A special failure detection system is used to shut the system down under predetermined conditions including failure of the motor, lack of flow through the electrolytic cell, or an open or short circuit through the cell.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic sectional view of the mechanical portion of the system, according to the invention, embodying the centrifugal impeller.

FIG. 2 is a plan view of the impeller of FIG. 1.

FIG. 3 is a sectional view taken along lines 3-3 of FIG. 2.

FIG. 4 is a block diagram of the electronic control system of the invention.

FIG. 5 is a schematic diagram illustrating the power supply, motor circuit, current power circuit and plating current sensor of FIG. 4 in more detail.

FIG. 6 is a schematic diagram illustrating the ion addition circuit of FIG. 4 in more detail.

FIG. 7 is a schematic diagram illustrating the ion subtraction input, integrator, standby, shutdown and current control circuits of FIG. 4 in more detail.

FIG. 8 is a schematic diagram illustrating the failure detection system of FIG. 4 in more detail.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates the mechanical unit incorporating the electrolytic cell and the mechanism for circulating the fixing solution through the cell. A base 10 supported by resilent feet 12 is attached to a removable housing 14 which encases the electrolytic metal ion recovery cell 16 and circulation mechanism when in operation. The electrolytic cell 16 includes a removable stainless steel cylinder 18 which forms the cathode and a hollow platinum coated titanium anode 20 coaxially arranged within the cathode 18. The ends of the eletrolytic cell 16 are defined by a pair of opposing nonmetallic end plates 22 and 24 each attached to the base 10. Each end plate 22, 24 contains a plurality of aligned holes in a circular pattern such that rods 26 (only one of which is shown in FIG. 1) may be passed therethrough. The ends of each rod 26 extending beyond the respective end plates are threaded to accept nuts 28. Tightening the nuts 28 on all of the rods 26 causes the end plates 22 and 24 to be sealingly seated against the circular ends of the cylindrical cathode 18. The anode 20 within the cell 16 is supported at one end which passes through a sealed opening in the end plate 22 which is rigidly attached to a manifold block 30 such that solution may be introduced through the hollow anode 20 via an intake hose 32 which is in fluid communication with the fixing solution bath (not shown). An outlet hose 34 is connected via the manifold block 30 to an offset exit port in the end plate 22 near the inner periphery of the cathode 18, through which solution is discharged from the cell 16 back to the fixing bath.

A motor 36 rigidly supported by the base 10 has an output shaft 38 which drives a cooling fan 40. The shaft 38 is further connected via coupling 42 to a second shaft 44 which extends through a sealed bearing 46 in the other end plate 24 of the cell 16 and is finally connected to a centrifugal impeller 48 coaxially mounted within the cell 16 between the open end of the anode and the end plate 24.

The impeller 48, as shown in more detail in FIGS. 2 and 3, includes a nonmetallic disc-shaped body 50 having a coaxial circular groove 52 facing the open end of the anode 20 (FIG. 1). The groove 52 has a conical inner wall 54 whose surface is inclined at a 45 angle relative to the axis of rotation and an outer cylindrical sidewall 56. Oppositely canted ports 58 and 60 are formed through the floor of the groove 52 diametrically opposite from each other to force fluid from the groove 52 to the rear of the impeller 48 to eliminate air bubbles which would otherwise form and remain in the re gion between the impeller 48 and the end plate 24. Air bubbles can interfere with the sealed bearing 46 which functions properly when it is completely wet.

Six equally spaced ports 62 are formed in the sidewall 56 of the groove 52. When the impeller is rotating, fluid is drawn through the ports 62 and respective ducts 64 formed through the impeller body 50 and exhausted through respective exit ports 66 equally spaced about the cylindrical periphery of the impeller body 50. The ducts 64 are directed approximately tangentially away from the circular groove 52 in the plane of rotation of the impeller 48 such that fluid is centrifugally forced through the ducts 64 and out the exit ports 66 when the impeller is rotating in the direction indicated in FIG. 2.

The rotation of the impeller 48 causes ion-laden solution entering the electrolytic cell 16 (FIG. 1) through the hollow anode 20 to be redirected toward the cylindrical cathode l8 creating a homogenous solution in the vicinity of the inner sidewall of the cathode. The action of the liquid in turn creates a vortex which produces a radial pressure gradient in the cell 16. The pressure at the axis or center line of the cell is less than the pressure at the cathode wall, and thus differential is the mechanism by which fluid is drawn through the hollow anode 20 and forced out along the cathode l8 and ultimately through the exit hose 34. The need for an external pumping means is thus eliminated completely. The horizontal mounting arrangement for the unit lowers its center for gravity and permits convenient location of the unit under a table, for instance, without interfering with the fluid vortex and plating uniformity.

The functional block diagram of the electronic control system for the metal recovery unit illustrated in FIG. 1 is shown in FIG. 4. The functions of the control system, specifically adapted for photographic fixing solutions, are to vary the plating current through the cell 16 in proportion to the metal ion concentration and to turn off the cell motor 36 and the plating current when the ion concentration is zero, referred to as the standby" mode, or when a predetermined failure condition has been detected, such as a short circuit through the electrolytic cell 16.

The heart of the system responsible for maintaining the proper level of the plating current is an integrator circuit 68 receiving control inputs representing l) the rate at which silver is being plated onto the cathode 18 in the cell 16 and (2) the rate of introduction of silver ion bearing film. A plating current sensor 70 provides the integrator input via an ion subtraction input circuit 72 representing the rate at which silver is being removed from the cell 16. A processor feed switch 74 provides a constant signal input to the integrator circuit 68 via an ion addition input circuit 76 during the time that fresh film is added to the photographic processing system. The assumption is made that film is mechanically introduced into the processing system at a uniform rate such that the total time taken in advancing a standard size film into a processor is directly proportional to the quantity of silver ions introduced thereby into the solution. The output of the integrator circuit 68 represents the time integral of the rate at which silver ions are added to the solution less the rate at which silver ions are removed from the system by plating. Hence the integrator circuit output is proportional to the net silver ion concentration in the cell at any given time. The output of the integrator circuit 68 is passed via a plating current control circuit 78 to control a power circuit 80 which supplies plating current to the cell 16 in accordance with the output of the integrator circuit 68.

After a period of time following the last introduction of film with silver ions into the solution, the silver ion concentration in the cell 16 will be reduced to zero, barring the addition of new film into the processor. At this point, the plating current should be shut off and the motor 36 (FIG. 1) circulating the fixing solution should be stopped to preserve its bearings. A standby circuit 82 monitoring the output of the integrator circuit 68 senses a minimum output of the integrator circuit, which will always be slightly above zero because it should be set to lag the ion concentration slightly and can only be assymptotic to zero. The standby circuit 82 causes the integrator output to be reset to zero and issues an output command to a shut-down circuit 84 which turns off the plating current by means of the plating current control circuit 78 and shuts off the motor by means of a cell motor relay 86. A standby override circuit 88 causes the control system to resume operation when new silver ions are added to the cell 16 as sensed by the processor feed switch 74.

A failure detection system 90 is also used to signal the shutdown circuit 84 to turn off the plating current and the cell motor in the event of the occurrence of specific failures. There are four modes of failure: (1 a lack of flow of liquid through the cell 16, (2) a short circuit between the electrodes of the cell 16, (3) an open circuit between the electrodes, and (4) failure of the motor 36. A central fail detector 92 receives inputs from a cell flow sensor 94, a cell short circuit sensor 96 and open circuit sensor 98 and an r.p.m. sensor 100. The failure signal from the fail detector 92 is inhibited by a fail override circuit 94 during the standby mode when the motor and plating current are supposed to be off.

FIGS. 5-8 illustrate a specific embodiment of the control system according to the invention. In FIG. 5 a DC. power supply 102 includes a source 104 of standard 120 volt alternating current which is connected via a master switch 106 to a transformer 108 providing a low voltage output (e.g., 10 volts) to a full wave rectifier bridge circuit 110. The output of the bridge circuit 110 is smoothed by the resistive-capacitive (RC) integrator circuit 112 and passed as the unregulated voltage, V,,, to an integrated circuit voltage regulator l 14. The unregulated voltage V is nominally 10 volts and the regulated voltage output of the regulator 114, V is 6 volts, for example.

The motor 36 spinning the impeller 48 in the cell 16 receives power from the volt source 104 ia the master switch 106 and the motor relay 86 which is energized by the voltage V while processing is carried on. A manual override switch 116 is provided to energize the motor if desired when the system is in a standby or failure mode.

The power circuit 80 (FIG. comprises a pair of dual power transistors 118 and 120 connnected as illustrated to the unregulated voltage V The plating current output of the power circuit 80 is connected to the anode in the cell 16. The bases of the tandem power transistors 118 and 120 are connected in common to receive the control signal from the current control circuit 78 in order to vary the level of plating current.

The plating current sensor 70 includes a grounded sense resistor 122 connected to the cathode 18 of the cell 16. The feedback signal on lead 124, passed to the ion subtraction input 72 of the integrator circuit 68 is connected in parallel with the sense resistor 122 to the cathode 18 to develop a voltage signal proportional to the plating current. A meter 126, calibrated in Troy ounces, is connected in parallel with the sense resistor 122 to guage the quantity of silver plated on the cath ode 18 in the cell 16. A conventional bubble" electrolytic cell may be used for meter 126.

The metal ion addition input circuit 76, shown in FIG. 6, includes an opto-isolator unit 128 having a lamp 130 connected to the processor feed switch 74 (FIG. 4, e.g. a microswitch) which indicates that film is being advanced in the processor. When this switch is closed, voltage is provided to energize the lamp 130 in the opto-isolator. The light from the lamp is detected by a photoresistor 132. The output signal from the opto-isolator 128, connected as illustrated, is transmitted through transistor switching amplifier 134 and 136 and an input rate setting potentiometer 138 to the integrator circuit 68 (FIG. 7). A light emitting diode (LED) 140 is connected as illustrated to the output of the tran sistor 134 to indicate that film is being loaded into the processor. When the film has been advanced through the processor and no new film has been added, the processor feed switch 74 opens turning off the lamp 130. Interrupting the output of the opto-isolator 128 turns off the transistors 134 and 136 and the LED. 140. Accordingly, the output of the ion addition input circuit 76 is a step function which is at a particular level set by the potentiometer 138 when film is being advanced in the processor.

In FIG. 7 the integrator circuit 68 includes an opera tional amplifier 142 connected as illustrated in the form of a conventional integrator circuit. The output of the metal ion addition input circuit 76, taken from the potentiometer 138 is connected as illustrated to one input of the operational amplifier 142. The same input of the operational amplifier is connected in parallel to the metal ion subtraction input circuit 76 which feeds the plating current signal on lead 124 to the integrator via a potentiometer 144 which is used to set the removal" rate. The rate of change of the concentration of metal ions in the solution depends, of course, on the volume of the solution. Thus, the input rates of metal ion addition and subtraction must be established beforehand by means of the respective potentiometers 138 and 144 in the input circuits 76 and 72. For this purpose, the potentiometers 138 and 144 may be calibrated in terms of volume, for example.

The integrator circuit 68 includes a special capacitive element 146 connected across the output and the signal input of the operational amplifier 142 to form an analog integrator. The capacitive element 146 is preferably an energy storage cell" (manufactured by Ionics Division of Gould, Inc.) which functions in a manner similar to a conventional capacitor. The voltage across the element 146 increases with charge up to a predetermined maximum voltage whereupon the voltage across the device remains constant with further increases in charge. When the device 146 is discharged, the voltage will again remain constant until the same amount of charge has been discharged therefrom which was applied thereto during the constant voltage segment of its operation. With the futher removal of charge from the device 146, the voltage will decrease again following the same curve that it followed when the voltage was increasing. One advantage of using the special capacitive element 146 instead of a standard capacitor, whose voltage is limited only by dielectric strength, lies in the fact that the variable current source does not have unlimited current capacity but will provide plating current up to a predetermined maximum plating current. The special element 146 makes additional signal limiting circuitry unnecessary. Also the large effective capacitance that the energy storage cell provides makes the integrator circuit considerably simpler.

A relay switch 148 is connected in series with element 146 and is closed when energized by the unregulated voltage V,,. Thus when the master switch 106 (FIG. 5) is opened or power fails, the element 146 retains its charge so as to memorize the last calculation of the integrator circuit 68. Another relay switch 150 is connected in parallel with the capacitive element 146 so as to discharge the element 146 when the standby mode occurs at zero ion concentration. The dissipation of whatever charge is left on element 146 by means of the relay 150 insures that when new film is advanced into the processor the integrator output will start increasing from zero. Otherwise, errors would accumulate over a period of time covering a number of instances in which the standby mode was entered and the plating current shut off. The other input to the amplifier 142 is connected as illustrated via lead 152 to a reference voltage.

An integrator manual adjustment circuit 153 includes an integrator balance potentiometer 155 connected to the signal input to the operational amplifier 142 providing a common setting function to zero both the ion input rate and the ion removal rate which are independently set by the potentiometers 138 and 144 respectively. The signal input to the amplifier 142 is also connectible to the regulated voltage V through switch 157 and to ground through switch 159 connected through resistors as illustrated in order to artificially increase or decrease the signal input to the amplifier and thus the plating current, if desired, for example, during testing.

The current control circuit 78 includes an operational amplifier 154 which is connected as illustrated to receive the output of the integrator circuit 68, that is, the output of the operational amplifier 142. The operational amplifier 154 also receives a negative feedback signal voltage from the cathode 18 of the cell 16. This negative feedback signal voltage corresponds in amplitude to the amplitude of the current flowing through the cell 16 and serves to maintain the cell current constant for the given output signal voltage from the integrator 68 even though the load represented by the cell 16 changes. Thus, the plating current control circuit and the power circuit 80 function in combination as a variable current source to apply a current to the cell 16 9 corresponding to the output signal voltage of the integrator circuit 68 regardless of the load represented by the cell 16.

A potentiometer 156 connected as illustrated between a reference voltage and one input of the operational amplifier 154 provides a means for setting the output of the operational amplifier 154 and the current applied to the cell 16 to the correct level so that when the output of the integrator circuit 68 is zero, the plating current through the cell will also be zero.

The standby circuit 82 has another operational amplifier 158 connected as illustrated to receive the output of the integrator circuit 68. The function of the standby circuit 82 is to compare the output of the integrator circuit 68 to a reference level in order to detect the pre-established minimum output of the integrator circuit 68 which corresponds to a zero ion concentraton. The minimum level is set by means of a potentiometer 160 intercepting the output of the integrator circuit 68 to the standby circuit amplifier 158. The output of the standby operational amplifier 158 indicative of the predetermined minimum level for the integrator output is connected as illustrated to a transistor 162. When the standby mode occurs, the transistor 162 causes the relay switch 150 in the integrator circuit 68 to be energized, thus discharging the capacitive element 146 in order to reset the integrator to zero. The transistor 162 is also connected to energize an LED. 164 which, when lighted, indicates the standby mode.

The output of the standby amplifier 158 is connected in parallel to the input to the shut-down circuit 84 whose function is to shut off the plating current and the motor 36. The circuit 84 includes a simple operational amplifier 166, which may be implemented by a Norton amplifier, connected as illustrated to transmit the output of the standby amplifier 158. The output of the shut-down amplifier 166 is connected via diodes 168 to transistor amplifiers 170 and 172 which are in the con ducting state when the electrolytic cell is in operation, that is before the standby mode is reached. An LED. 174, connected as illustrated to the output of the transistor 170, is lighted to indicate the processing mode. When the standby signal is transmitted via the shutdown amplifier 166, the diodes 168 are biased 01f thus causing the voltage to rise at the base of transistor 170 cutting it and transistor 172 off along with the LED. 174. As a result, the voltage at the output of transistor 172 rises to the unregulated voltage V which causes the motor relay 86 (FIG. 5) to be de-energized since the other side of the relay coil is also connected to V,,. De-energizing the motor relay 86 causes the motor circuit to be opened. The motor 36 is automatically shut off in the standby mode to preserve its bearings so that a less expensive motor may be employed. The output of the shut-down amplifier 166 is also passed to the current control circuit 78 via the diode 175 to the input of the current control amplifier 154 to set its output at zero thus cutting off the plating current altogether.

The standby override circuit 88 receives the input signal P which is the output of the transistor 134 (FIG. 6) in the ion addition input circuit 76. In the override circuit 88, the lead with the input signal P is connected to a transistor 178 interconnected as illustrated with the transistor 162 in the standby circuit 82. When the signal P indicative of film loading first occurs, the tran sistor 178 will be turned on so as to turn off the transistor 162 in the standby circuit. This condition will remain as long as film is being loaded. Turning off the transistor 162 in the standby circuit causes the reset relay in the integrator circuit to be de-energizcd to return to the open condition and allow the capacitor element 146 to receive charge via the metal ion addition input circuit 76. This allows the film input to start the integrator. Once the integrator has started charging, the standby circuit 82 will hold the reset relay 150 deenergized. The series capacitor 176 and resistor connected to the base of the transistor 178 allows the machine to recover from a power failure or power off condition. When power fails, the last calculation value is preserved in cell 146 because relay 148 opens immediately upon power failure. When circuit power returns, current will initially flow in capacitor 176 as V rises. Thus transistor 178 is rendered conductive to keep transistor 162 turned off and the relay switch 150 from closing during power-up transients. When the capacitor 176 in the override circuit 88 becomes charged. the transistor 178 is no longer able to cut off the transistor 162 in the standby circuit 82. However, the capacitor 176 is chosen to provide a delay so that the standby circuit is disabled for a sufficient period of time to allow the circuit to stabilize and power-up transients to cease. The capacitor 176 thus functions to prevent the system from losing its memory as a result of power failure and the system will resume operation at the same point at which it was interrupted when power failed.

The failure detection system 90, shown in FIG. 8, includes four resistors 180, 182, 184 and 186 receiving respective error signals connected in parallel to the input of a simple operational amplifier 188, which may again be implemented with a Norton amplifier. A capacitor 190 connected around the amplifier 188 provides an integration effect which requires persistence of a given fail signal for a set time interval before an output signal is produced by the amplifier 188.

The resistor 180 in the detection system is connected to the cell short circuit error detector 96 which includes an operational amplifier 194, which may be 21 Norton amplifier, connected as illustrated to the cathode 18 and anode 20 of the cell 16. If the cell should become shorted, this will cause a drop to zero in voltage across the cell which will be detected by the short detector amplifier 194. The open circuit detector 98 includes an operational amplifier 196 connected as illustrated to the anode 20 of the cell 16. The power circuit 80 will provide the current determined by the control circuit 78 to the cell 16 regardless of the load provided by the cell. As a result an open circuit in the cell will cause a rise in voltage at the anode 20 which will be detected by the open circuit amplifier 196. The outputs of the short circuit and open circuit amplifiers 194 and 196 are connected in common to an indicator circuit comprising a transistor and an LED. 197 connected as illustrated to provide a common indication of a cell error of either type. The r.p.m. sensor 100 includes an input from a photocell (not shown) detecting light reflected from a pattern on the drive shaft of the motor 36 to produce an AC. signal input via a decoupling capacitor 198 to a transistor 200 whose output is connected in parallel to a low pass capacitor 202 which converts the output to a DC. signal indicative of r.p.m. passed to the sense resistor 184.

The cell flow sensor 94 provides an output to resistor 186 indicative of a lack of flow in the cell 16. The cell flow sensor output is connected in parallel to an indicator circuit comprising a transistor 204 and an LED.

1 1 206 connected as illustrated to provide a no-flow indication.

A means must be provided for distinguishing the standby mode from a failure condition because in the standby mode the motor is not running. A persistent error signal will be generated by the rpm. sensor 100 as well as the cell flow sensor 94 in the standby mode. Hence, a fail override circuit 94 is interconnected between the standby circuit and the fail amplifier 188 as illustrated to cancel out the error signals. The standby override circuit 88 automatically cancels the disablement of the fail detection system when new film is fed into the processor.

When a persistent bonafide error is detected, the fail signal is sent to the shut-down circuit 84 directly as the output of the fail amplifier 188 (not via the diodes 208 and 210) to shut off the motor and plating current without resetting the integrator 68. The output of the shutdown circuit amplifier 166 (FIG. 7) is connected as illustrated via diode 210 to the junction of opposed diodes 208 and 212, which form with diode 210, an AND gate. in the absence of a fail signal, at the output of the amplifier 188, the shut-down circuit signal, produced by the standby mode is shunted by the diode 208 because it is not reverse biased. When a fail signal is present, diode 208 becomes reversed biased and the shutdown signal, which in this case would be due to the failure mode, is routed via the diodes 212 to the transistor 214 to light up the L.E.D. 216 indicating failure. An audible alarm, connected as illustrated, can also be used to signal a failure.

In practice, all of the electronic circuitry in FIGS. 6, 7 and 8 as well as the integrated circuit voltage regulator 144 in FIG. are provided together on a single printed circuit board of the plug-in type. The other electronic components including the remainder of the power supply and the variable current source would normally be provided separately on a heat sink.

Those skilled in the art will recognize that the implementation of the functional principles of the invention can be accomplished using all digital instead of analog circuitry. Many other modifications and adaptations of the circuitry disclosed are of course possible without departing from the principle of the invention. Thus, the above-described embodiment is intended to be illustrative and in no way restrictive regarding implementation of the system. The scope of the invention is indicated by the appended claims and all variations and adaptations which come within the range of equivalence thereof are intended to be embraced therein.

I claim:

l. A system for recovering silver from a chemical fixing solution in a photographic film processing system comprising an electrolytic cell having an anode and cathode adapted to be immersed in the chemical fixing solution of said film processing system, a variable current source operatively connected to the electrodes of said electrolytic cell to provide plating current therethrough, computing means to continuously compute the level of silver ions in said fixing solution from the amplitude of said plating current and the amount of film processed by the system and to generate an indication representing said level of said ions, said computing means changing the value of said indication in one direction in response to film being processed in said film processing system and changing the value of said indication in the opposite direction responsive to the amplitude of plating current flowing in said electrolytic 12 cell the rate of change of said indication corresponding to said amplitude of plating current, and means to control said variable current source in accordance with said indication generated by said computing means to vary said plating current through a plurality of levels in accordance with the value of said indication.

2. A system as recited in claim 1, wherein said means to control said variable current source causes said current source to vary said plating current proportionally to variations in said indication.

3. A system as recited in claim 1, wherein said variable current source is variable throughout a continuous range of current values and said means to control said variable current source is operable to cause said variable current source to have any value in said continuous range depending upon the value of said indication.

4. A system as recited in claim 1, wherein said variable current source, said computing means, and said means to control said variable current source comprise means to vary said plating current proportionally to the concentration of silver ions in said solution up to a maximum value and to maintain said plating current at said maximum value for silver concentrations greater than the concentration corresponding to said maximum value of plating current.

5. A system as recited in claim 1 including means to shut off said plating current applied by said variable current source to said electrodes in response to said indication reaching a predetermined value indicating that the silver ion concentration in said solution has dropped to a predetermined concentration.

6. The system of claim 1 further comprising standby means responsive to said output indication reaching a predetermined value representing a minimum level of ion concentration for automatically resetting said output indication to a value to represent zero ion concentration.

7. The system of claim 6, further comprising a standby override circuit for disabling said standby means for a predetermined length of time upon the adding of new metal ions to the solution.

8. A system as recited in claim 1 wherein said computing means is an analog integrator having a charge storage device, the output voltage of said analog integrator depending upon the voltage across said charge storage device and comprising said output indication of said computing means, said analog integrator changing the charge on said charge storage device in one direction in proportion to the integral of said plating current and changing the charge on said charge storage device in the opposite direction at a constant rate whenever film is being processed by the fixing solution in said photographic cilm processing system.

9. A system as recited in claim 8 wherein said analog integrator further comprises an operational amplifier, said charge storage device being connected between the output and an input of said operational amplifier and means to apply signal proportional to said plating current to an input of said amplifier and to apply a signal with a constant amplitude to an input of said operational amplifier whenever film is being processed by the fixing solution in said photographic film processing system.

10. A system as recited in claim 1 wherein pump means are provided to continuously circulate fixing solution from said film processing system to said electrolytic cell.

11. A system as recited in claim further comprising means responsive to the failure of said pump means to operate to interrupt said plating current.

12. A system as recited in claim 10 further comprising means to detect the flow of said fixing solution through said electrolytic cell and means responsive to said flow sensing means indicating a lack of flow of fixing solution to interrupt said plating current.

13. A system as recited in claim 1 further comprising means responsive to said plating current rising above a predetermined value to interrupt said plating current.

14. A system as recited in claim 10 further comprising means responsive to said plating current falling below a predetermined minimum value to interrupt said plating current.

15. An electrolytic plating system comprising an electrolytic cell having an anode and cathode adopted to be immersed in a chemical solution bearing metal ions, a variable current source connected to said cell to apply plating current therethrough, means producing a control signal indicative of the instantaneous concentration of metal ions in said solution up to a predetermined concentration of metal ions and to hold the value of said control signal constant for ion concentrations greater than said predetermined concentration, and means responsive to said control signal for continuously controlling said current source to apply said plating current with an amplitude related to the value of said control signal.

16. An electrolytic plating system comprising an electrolytic cell having an anode and a cathode adapted to be immersed in a chemical solution bearing metal ions, a variable current source connected to said cell to apply plating current therethrough, computing means to continuously compute the integral of the rate said ions are added to said solution minus the rate said metal ions are plated out of said solution, said integral representing the metal ion concentration in said solution, means responsive to said integral for controlling 14 said variable current source to apply said plating current with an amplitude related to said integral, and standby means responsive to the value of said integral falling below a predetermined minimum level for automatically resetting said integral to a value to represent zero ion concentration in said chemical solution.

17. The system of claim 16, further comprising a standby override circuit for disabling said stand by means for a predetermined length of time upon the adding of new metal ions to the solution.

18. A system for recovering silver from a chemical fixing solution in a photographic film processing system comprising an electrolytic cell including an anode and a cathode, means to circulate the chemical fixing solution from said photographic film processing system continuously through said electrolytic cell, a variable current source operatively connected to the electrodes of said electrolytic cell to provide plating current the rethrough, signalling means to indicate when film is being processed by said film processing system to add silver ions to the fixing solution of said film processing system, plating current detection means to detect said plating current amplitude and to generate a signal representing the amplitude of said plating current, integrating means to continuously compute the level of silver ions in said fixing solution and generate a control signal representing the level of concentration of silver ions in said fixing solution by integrating a constant value to change said control signal in one direction in response to said signalling means indicating that film is being processed by said film processor and by integrat ing the value of the output signal of said plating current detection means to change the value of said control signal in the opposite direction, and means responsive to the value of said control signal to control said variable current source to apply a plating current to said electrodes having a value related to the value of said control signal. 

1. A SYSTEM FOR RECOVERING SILVER FROM A CHEMICAL FIXING SOLUTION IN A PHOTOGRAPHIC FIILM PROCESSING SYSTEM COMPRISING AN ELECTROLYTICD CELL HAVING AN ANODE AND CATHODE ADAPTED TO BE IMMERSED IN THE CHEMICAL FIXING SOLUTION OF SAID FILM PROCESSING SYSTEM, A VARIABLE CURRENT SOURCE OPERATIVELY CONNECTED TO THE ELECTRODES OF SAID ELECTROLYTIC CELL TO PROVIDE PLATING CURRENT THERETHROUGH, COMPUTING MEANS TO CONTINUOUSLY COMPUTE THE LEVEL OF SILVER IONS IN SAID FIXING SOLUTION FROM THE AMPLITUDE OF SAID PLATING CURRENT AMOUNT AND THE AMOUNT OF FILM PROCESSED BY THE SYSTEM AND TO GENERATE AN INDICATION REPRESENTING SAID SAID INDICATION IN ONE DIRECTION IN RESPONSE TO FILM BEING PROCESSED IN SAID FILM PROCESSING SYSTEM AND CHANGING THE LEVEL OF SAID IONS, SAID COMPUTING MEANS CHANGING THE VALUE OF SAID INDICATION IN ONE DIRECTION IN RESPONSE TO FILM BEING PROCESSED IN SAID FILM PROCESSING SYSTEM AND CHANGING THE VALUE OF SAID INDICAATION IN THE OPPOSITE DIRECTION RESPONSIVE TO THE AMPLITUDE OF PLATING CURRENT FLOWING IN SAID ELECTROLYTIC CELL THE RATE OF CHANGE OF SAID INDICATION CORRESPONDING TO SAID AMPLITUDE OF PLATING CURRENT, AND MEANS TO CONTROL SAID VARIABLE CURRENT SOURCE IN ACCORDANCE WITH SAID INDICATION GENERATED BY SAID COMPUTING MEANS TO VARY SAID PLATING CURRENT
 2. A system as recited in claim 1, wherein said means to control said variable current source causes said current source to vary said plating current proportionally to variations in said indication.
 3. A system as recited in claim 1, wherein said variable current source is variable throughout a continuous range of current values and said means to control said variable current source is operable to cause said variable current source to have any value in said continuous range depending upon the value of said indication.
 4. A system as recited in claim 1, wherein said variable current source, said computing means, and said means to control said variable current source comprise means to vary said plating current proportionally to the concentration of silver ions in said solution up to a maximum value and to maintain said plating current at said maximum value for silver concentrations greater than the concentration corresponding to said maximum value of plating current.
 5. A system as recited in claim 1 including means to shut off said plating current applied by said variable current source to said electrodes in response to said indication reaching a predetermined value indicating that the silver ion concentration in said solution has dropped to a predetermined concentration.
 6. The system of claim 1 further comprising standby means responsive to said output indication reaching a predetermined value representing a minimum level of ion concentration for automatically resetting said output indication to a value to represent zero ion concentration.
 7. The system of claim 6, further comprising a standby override circuit for disabling said standby means for a predetermined length of time upon the adding of new metal ions to the solution.
 8. A system as recited in claim 1 wherein said computing means is an analog integrator having a charge storage device, the output voltage of said analog integrator depending upon the voltage across said charge storage device and comprising said output indication of said computing means, said analog integrator changing the charge on said charge storage device in one direction in proportion to the integral of said plating current and changing the charge on said charge storage device in the opposite direction at a constant rate whenever film is being processed by the fixing solution in said photographic cilm processing system.
 9. A system as recited in claim 8 wherein said analog integrator further comprises an operational amplifier, said charge storage device being connected between the output and an input of said operational amplifier and means to apply signal proportional to said plating current to an input of said amplifier and to apply a signal with a cOnstant amplitude to an input of said operational amplifier whenever film is being processed by the fixing solution in said photographic film processing system.
 10. A system as recited in claim 1 wherein pump means are provided to continuously circulate fixing solution from said film processing system to said electrolytic cell.
 11. A system as recited in claim 10 further comprising means responsive to the failure of said pump means to operate to interrupt said plating current.
 12. A system as recited in claim 10 further comprising means to detect the flow of said fixing solution through said electrolytic cell and means responsive to said flow sensing means indicating a lack of flow of fixing solution to interrupt said plating current.
 13. A system as recited in claim 1 further comprising means responsive to said plating current rising above a predetermined value to interrupt said plating current.
 14. A system as recited in claim 10 further comprising means responsive to said plating current falling below a predetermined minimum value to interrupt said plating current.
 15. An electrolytic plating system comprising an electrolytic cell having an anode and cathode adopted to be immersed in a chemical solution bearing metal ions, a variable current source connected to said cell to apply plating current therethrough, means producing a control signal indicative of the instantaneous concentration of metal ions in said solution up to a predetermined concentration of metal ions and to hold the value of said control signal constant for ion concentrations greater than said predetermined concentration, and means responsive to said control signal for continuously controlling said current source to apply said plating current with an amplitude related to the value of said control signal.
 16. An electrolytic plating system comprising an electrolytic cell having an anode and a cathode adapted to be immersed in a chemical solution bearing metal ions, a variable current source connected to said cell to apply plating current therethrough, computing means to continuously compute the integral of the rate said ions are added to said solution minus the rate said metal ions are plated out of said solution, said integral representing the metal ion concentration in said solution, means responsive to said integral for controlling said variable current source to apply said plating current with an amplitude related to said integral, and standby means responsive to the value of said integral falling below a predetermined minimum level for automatically resetting said integral to a value to represent zero ion concentration in said chemical solution.
 17. The system of claim 16, further comprising a standby override circuit for disabling said standby means for a predetermined length of time upon the adding of new metal ions to the solution.
 18. A system for recovering silver from a chemical fixing solution in a photographic film processing system comprising an electrolytic cell including an anode and a cathode, means to circulate the chemical fixing solution from said photographic film processing system continuously through said electrolytic cell, a variable current source operatively connected to the electrodes of said electrolytic cell to provide plating current therethrough, signalling means to indicate when film is being processed by said film processing system to add silver ions to the fixing solution of said film processing system, plating current detection means to detect said plating current amplitude and to generate a signal representing the amplitude of said plating current, integrating means to continuously compute the level of silver ions in said fixing solution and generate a control signal representing the level of concentration of silver ions in said fixing solution by integrating a constant value to change said control signal in one direction in response to said signalling means indicating that film is being processed by said film processor and by integrating the value of The output signal of said plating current detection means to change the value of said control signal in the opposite direction, and means responsive to the value of said control signal to control said variable current source to apply a plating current to said electrodes having a value related to the value of said control signal. 